A spiral machine of the above-mentioned type is known from Dvorak et al. U.S. Pat. No. 3,600,114 wherein the embodiment shown in FIGS. 8 and 9 shows a two-speed, single-stage engine, in which the two mobile displacer disks are mounted loosely on stationary eccentric axles. One of the axles is hollow to conduct the working medium to be transported out of the engine. On their circumference, the displacer disks are equipped with gear rims which are engaged by a common gear wheel mounted on a drive shaft. These multispeed engines have the advantage that, on the one hand, each of the displacer disks is completely balanced in itself, and on the other, that a more uniform conveyance almost without pulsation is possible. In addition, the radial displacement of the two disks and, thus, the eccentricity between the two rotating axles is smaller than with single speed engines which leads to lower sliding velocities between the helical ribs. In principle, therefore, operation with higher revolutions per minute are allowed with this type of supercharger. However, at these higher rpm's the strength of the ribs presents a problem.
In the aforecited known engine, the strength of the ribs has been taken into account in that the ribs extend in the shape of a trapezoid from top to bottom. However, this solution is advantageous only in the case of spiral chargers transporting low volumes, that is, in the case in which the axial length of the ribs is small.
In addition to utilizing the aforementioned trapezoidal configuration of the ribs, it is known from U.S. Pat. No. 2,324,168 in spiral machines of the aforementioned type to also configure the cross-section of the ribs to be variable in order to supplement strength. It is proposed, for example, to shape the inner wall to both of the cooperating spirals in a purely archimedean manner while the outer walls exhibit a non-constant, increasing rise with growing angles of contact. This leads to spirals with wall thicknesses increasing from the inside out. The measure is intended primarily to obtain an improved sealing effect at the points of contact of the two spirals travelling along the helix. As a result of such configuration, the spirals on the outer periphery, i.e., in the area in which the outer wall of the spiral is no longer needed to form a conveying space and thus does not have to perform a sealing function, are too "thick", and, hence, it is recommended to configure the wall on the outer spiral part to be thinner relative to the spiral parts located further inward. Spirals with variable wall thicknesses therefore have conveying chambers wherein the walls are not parallel. Consequently, the chambers cannot be made by turning.
In the case of spiral chargers for supercharging, the large volume of media to be conveyed requires wide conveying chambers. The ribs are therefore usually formed by helical ridges that are essentially vertical and that have a larger axial length relative to their thickness. The vertical terminal edges of the ridges are thus relatively unstable at least in the area of the fiber farthest from the displacer disk, i.e. in the head region. Consequently, in operation, the terminal edges could strongly impact the foot parts of the cooperating ridges. In addition, there is significant stressing in the foot area of these terminal edges, which may even lead to fracture.
It is therefore the object of the invention to create a spiral charger of the aforementioned type in which the deformation by centrifugal forces of the ribs is largely prevented.
This object is attained utilizing ribs that ar provided on their outer periphery with reinforcements which are located on the outer wall of the ribs, which extend at least approximately over the entire axial length of the ribs, and which extend in the circumferential direction at most from the end of each rib at the inlet side to the end on the inlet side of the rib following it in the circumferential direction without affecting the inlet cross-section of the conveying space formed by the two ribs.
The advantage of the invention is to be found particularly in that the spiral parts through which the working medium is flowing can be made with the lowest possible wall thickness. With constant eccentricity, this signifies a gain in space that increases with the magnitude of the contact angle of the spirals.
It is known from EP-A-275415 to reinforce the ribs at the rib end on the inlet side within an angular range of 0.degree. to 120.degree. in order to protect the inlet edge of the ribs in the area of the transition to the displaced disk. Preferably, the wall thickness is to increase gradually to a maximum size at the inlet edge itself. This continuous increase is advantageously effected by a helical expansion of the outer contour relative to the inner contour or conversely by the helical decrease of the inner contour relative to the outer contour. However, this known machine is of a type in which the displacer disk is orbiting in a stationary spiral housing, which is in contrast to the present machine in which two displacer disks are rotating together. Aside from the varying manufacturing problems involved, this known measure obviously requires that the contours of the stationary spiral housing cooperating with the displacer disk also be adapted to the variable helical shape of the displacer ribs.